Softstarter tools are commonly known in the prior art. They are used to start or stop a motor avoiding electrical surges, sudden overheating of motors, mechanical stresses and shocks, wearing out material and mechanical damage in the driven apparatus.
EP 1 914 875 A1 discloses a motor starter device adapted for controlling an electric motor connected to the device especially during a start or a stop of the motor. The prior art softstarter device comprises calculating means for calculating a difference between a determined motor torque and a reference value for the motor torque, and calculating an error signal from the comparison between said calculated torque and the reference value to change the speed of the motor. The softstarter device also comprises means for reducing the speed of said motor and the rate of change of motor speed with respect to time during a stopping time interval. The speed of said motor is reduced dependent on said error signal so that the rate of change of motor speed with respect to time during a stopping time interval varies at least between a first part of the time interval and a second part of the time interval.
When the electric motor load is changed, the softstarter should be adjusted to conform to the requirements of the new aggregates. This is often a tedious and time-consuming procedure and—in the worst case—it may be forgotten. The aggregates which are to be driven by the electric motor may sometimes undergo a change in the required torque due to changing of conditions (such as increasing or decreasing back pressure in a pump or compressor or material ageing).